my nikon 18-200mm is a "DX" lens, so as i understand it, it is designed to be used with a cropped sensor dslr...as the sensor is NOT the same size as a full 35mm film frame.... if this lens were mounted on a full frame camera would it be equivalent to a 27-300mm..... or am i missing something?

now, if i buy a lens designed for a 35mm or a full frame dslr, do i devide the listed spec by 1.5 to get the range for my cropped dslr?

A 300mm lens on a camera with a 1.5 cropped sensor is still a 300mm lens. The magnification will always be 300mm no matter what the sensor size. The field of view will alter however. It's field of view is equivalent to a 450mm lens on a full frame sensor.

In your example, the 18-200mm Nikkor at 18mm on a cropped sensor would have the same field of view as a 27mm lens on a full frame sensor. It will not magnify the image.

so a traditional 50mm lens used with a 1.5 crop = the field of view of a 75mm lens mounted on a full frame?

but a DX lens 12-24mm = 18-36mm on a full frame? or do you divide when using DX lens on a full frame?

i don't get it..

You do not divide for full frame (35 mm) - what is written on the lens, regardless of it being a DX lens or not - is what you will get on a full frame body. What a DX lens will cause on a full frame sensor is some serious vignetting...

OHHH OK... so a DX lens - or other crop sensor specific lenses (such as tamron di II lenses) provide the same field of view on crop and full frame sensors... but you don't want to use a crop sensor specific lens on a full frame body....

you multiply standard lenses only when mounting on crop sensor body's..

GREAT... thanks!

PS: tamron DI lenses (not DI II) are designed for full frame... right?

There is a quite detailed discussion of this topic in a Luminous Landscape Tutorial - click to follow the link. I think it will really help and doing so will help ease my conscience about copying some of the text from the article to label the picture below!.

So, on the assumption that a picture is worth a thousand words here is a picture and its accompanying description from that article just to whet your appetite.

The image circle created by a 35mm format lens is shown.The blue rectangle shows what a 35mm film, or a full-frame digital SLR will record.The inner red frame shows what a digital SLR with a 1.6X magnification factor will record.

so a DX lens - or other crop sensor specific lenses (such as tamron di II lenses) provide the same field of view on crop and full frame sensors

No! The croping depends on the body, not the lens!
And yes, the Tamron Di is for FF or smaller sensor bodies, the Di II is for APS-C or smaller format sensors. So if you use a Di II lens on a FF body, you will get dark corners or even dark borders

so a DX lens - or other crop sensor specific lenses (such as tamron di II lenses) provide the same field of view on crop and full frame sensors

No! The croping depends on the body, not the lens!And yes, the Tamron Di is for FF or smaller sensor bodies, the Di II is for APS-C or smaller format sensors. So if you use a Di II lens on a FF body, you will get dark corners or even dark borders

i'm confused by your statement...
tamron di is for smaller sensor bodies, while the di II is for aps-c or small format sensors.... aren't aps-c sensors considered "small sensors" as they are smaller then full frame sensors?

Thanks, I am a newbie but I read the link and it make perfect sense. If I am not mistaken, 4:3 format is worse in this regard than other DLSR's. More like 2x than 1.6x. Is this correct? Then maybe a 4:3 is a bad choice for landscape?

What I don't understand is why 4:3 was chosen as the format for the new digital standard. Can anybody explain the benefits of it? Is it just because it scales to normal print sizes?

What I don't understand is why 4:3 was chosen as the format for the new digital standard.

From memory, and somebody please correct me if I've got it wrong, 4:3 was chosen because it was thought that most people would view their digital pictures on TV sets and/or computer monitors. Trouble was nobody at the time realised that wide-screen would soon become ubiquitous.

Yep, I think 4:3 was chosen as the native aspect ratio for compact digital cameras to match the shape of old-style TVs and PC monitors. It's interesting though that the Four Thirds DSLR system also adopted the 4:3 shape.

To be fair, most medium format cameras are also closer to this image shape...